Parameters of light include wavelength, intensity, as well as polarization, which represent the direction of an electric field of light and the change of the electric field over time. In optical communication networks, when signal light propagates in an optical fiber transmission line, the state of polarization changes under the effect of environmental conditions, such as temperature, and under conditions such as external stress. It is a known fact that optical fiber transmission lines and optical amplifiers have polarization dependence, which causes the transmission characteristics of optical fiber transmission lines to deteriorate.
Polarization dependence is primarily caused by polarization dependence gain (PDG) and polarization dependence loss (PDL), and cause characteristics, such as the optical signal to noise ratio (OSNR) or Q-factor, to deteriorate.
As a technique for reducing the above effects of polarization dependence gain and polarization dependence loss, there is a proposed polarization scrambler that aggressively changes the polarization state of signal light and brings the signal light into a non-polarized state (a state in which electric fields are uniformly distributed in all directions) on the transmitting side.
The polarization scrambler performs polarization scrambling on signal light and transmits signal light that has a randomly polarized state. As a result, the degradation in transmission quality caused by polarization dependence gain and polarization dependence loss is reduced.
An optical transmission technique using a polarization scrambler has been proposed (see, for example, Japanese Laid-open Patent Publication Nos. 2000-196523 and 09-149006).
A polarization scrambler in the related art generally has a scrambling frequency (a frequency at which polarization rotation is performed) ranging from several hundreds of kHz to 1 MHz. At the scrambling frequency, a polarization state becomes random.
As a technique for realizing high-capacity long-distance transmission, a coherent receiver technology has recently received attention. The development of an optical transmission system that uses the coherent receiver technology is proceeding.
For example, with an optical transmission system in the related art that has a transmission speed less than or equal to 10 Gbps, the polarization dependence gain and polarization dependence loss are large. However, it is difficult to update the optical transmission system in the related art with the coherent receiver technology, because a polarization scrambler in the related art may not be usable.
The reason for this is that the scrambling frequency (ranging from several hundreds of kHz to 1 MHz) of a polarization scrambler in the related art exceeds a frequency (several tens of kHz) at which polarization trackability is obtained for coherent reception. If a polarization scrambler becomes unusable, the effects of polarization dependence, that is, polarization dependence gain and polarization dependence loss, occurs and degradation of transmission quality worsens.
Accordingly, there is an increasing demand for a technique to efficiently suppress degradation in transmission quality caused by polarization dependence, in other words polarization dependence gain and polarization dependence loss, even if an optical receiving apparatus that has limited polarization trackability, such as in coherent reception, is used in an optical transmission system in the related art.